This invention relates to automatic gate crossing mechanisms. In particular, this invention relates to automatic railroad gate crossing mechanisms.
Prior art railroad crossing gates mechanisms are well known. Upon the approach of a train at a roadway crossing (where railroad tracks and a roadway intersect) these ubiquitous mechanisms automatically lower a gate across a highway or road impeding the passage of vehicular and/or pedestrian traffic across the railroad tracks. When the train passes, the mechanism automatically lifts the gate.
Most gate control mechanisms use a D.C. motor to raise and lower the gate arm because the operating characteristics of D.C. motors make them ideally suited for such an application. Most of these D.C. motors are small, fractional or low horsepower motors because the gate arm is counterweighted by an appropriate counterweight. The motor is typically coupled to the gate arm through a torque-multiplying transmission or gear box reducing the amount of torque required to rotate the arm between the raised and lowered positions.
In most gate control mechanisms, a series/shunt type D.C. motor is used. All D.C. motors are reversible, have a high starting and running torque and the armature voltage readily controls its speed. The D.C. motor is typically coupled to the gate arm through a torque-multiplying transmission or gearbox. Balancing the gate arm using a counterweight system reduces the power required to raise and lower the gate--even when using a torque-multiplying gearbox.
To lower a crossing gate, the D.C. motor of a railroad gate crossing mechanism, which is coupled to a transmission or gear box, causes a crossing gate held in an "up" position to rotate downward through a predetermined angular arc. Cam-actuated limit switches cut off power to the motor so as to limit the motors' travel as the gate lowers. To raise the gate, the motor rotation direction is reversed causing the transmission to reverse direction thereby lifting the gate upward. Other limit switches and/or cam-actuated switches stop the motor as the gate reaches its full "up" position. To reduce costs, gate travel direction control is preferably achieved by simply reversing the D.C. motor instead of using the aforementioned transmission and/or gearbox.
In the course of lowering the gate arm using the motor, after the motor is energized to lower the gate arm, the gate arm begins to rotate in a downward direction and soon begins to experience an increasingly strong, downward gravitational force. As the gate arm rotates it continues to accelerate under the influence of gravity to a point where assistance from the motor is no longer necessary. Well before the gate is fully horizontal, power is cut off from the motor and only only the force of gravity lowers the gate arm. In fact, the downward travel of a properly counterweighted gate arm accelerates at such a rate such that the gate arm can be damaged when the gate arm drops to its horizontal position, i.e. when the gate arm is substantially parallel to the roadway surface, if the downward falling gate arm is not braked to slow the gate arm.
Another characteristic of a D.C. motors is it's ability to dynamically brake itself.
A unique characteristic of a D.C. motor is that it will act as a generator, producing a D.C. output current and voltage across the armature, if the armature is mechanically driven by an external mechanical power source. Prior art gate control mechanisms use this characteristic of a D.C. motor to dynamically brake the falling gate arm when the gate arm drops to some predetermined angular inclination with respect to the ground. Dynamic braking in the downward gate-travel direction is achieved by monitoring the angle of inclination of the gate arm. At some predetermined angle of inclination of the gate arm, and when the motor begins to act as a generator, cam operated switches apply an impedance across the armature impressing an electrical (and hence mechanical) load upon the generator. By appropriately selecting the value of the impedance used to shunt the armature the electrical and mechanical load on the generator can be adjusted to adjust the amount of braking effect produced by the generator. In fact, using dynamic braking, the gate arm travel rate can be slowed whereby the gate arm is not damaged as it falls to its downward position.
While downward travel dynamic braking is well known, prior art gate control mechanisms do not provide any upward travel dynamic braking. It is well known that railroad crossing gates are frequently damaged or broken when in the "down" position. When a railroad crossing gate is broken off when in the down position, the previously balanced gate arm suddenly becomes unbalanced. The unbalanced gate arm system will accelerate at an uncontrolled rate to the up position simply because of the weight of the arm's counterweight. Uncontrolled upward acceleration of the gate arm mechanism can damage the gate arm mechanism as the mechanism is suddenly forced to stop in the "up" position. Uncontrolled upward acceleration of the gate can also be present in normal operation of so-called fail clear gate mechanisms. That is a gate mechanism in which when a failure occurs the over-counterweighted system forces the gate to the up position.
A gate control arm mechanism which brakes the travel of a gate arm in both the upward and downward directions so as to avoid inadvertent damage to a gate control arm mechanism that might be caused by sudden and uncontrolled upward acceleration would be an improvement over the prior art. Braking a gate control arm mechanism as it travels to an up position preferably limits the gate's control arm up velocity to a maximum rate below which the gate control mechanism will not be damaged.